Further Practical Techniques (OCR A-Level Chemistry A): Revision Notes

Further Practical Techniques

Introduction

Preparing, purifying, and determining the melting point of organic solids are essential practical skills in organic chemistry. These techniques build upon previously learned methods such as distillation and heating under reflux using Quickfit apparatus. This note covers three important purification and analysis techniques used when working with solid organic products.

Preparation of organic solids

When carrying out organic synthesis, you may need to prepare solid products such as aspirin, salicylic acid, or ethanoic anhydride. These preparations often involve using Quickfit apparatus for reactions under reflux conditions. After the reaction is complete, the solid product must be separated from the reaction mixture before it can be purified.

Filtration under reduced pressure

Purpose and principle

Filtration under reduced pressure is a technique used to separate a solid product from a solvent or liquid reaction mixture more efficiently than simple gravity filtration. By applying suction, the filtration process is significantly faster, and the solid product can be partially dried on the filter.

Equipment required

To carry out filtration under reduced pressure, you need the following apparatus:

• Büchner flask (thick-walled conical flask with side arm)
• Büchner funnel (porcelain or plastic funnel with perforated base)
• Pressure tubing (thick-walled rubber tubing)
• Filter paper (circular, sized to fit the Büchner funnel)
• Access to vacuum pump or water pump

Procedure

Setting up the apparatus: Connect one end of the pressure tubing to the side arm of the Büchner flask and the other end to the vacuum outlet (or water pump tap). Fit the Büchner funnel securely to the top of the Büchner flask using a rubber bung or Büchner ring to ensure an airtight seal.

Preparing for filtration: Switch on the vacuum pump (or turn on the tap if using a water pump) and test for good suction by placing your hand over the top of the funnel. You should feel strong suction pulling air through.

Washing and drying: After all the mixture has been filtered, rinse the beaker with fresh solvent to collect any remaining solid crystals. Rinse the crystals in the Büchner funnel with additional solvent to remove impurities. Leave the crystals under suction for several minutes to remove excess solvent and begin the drying process.

Recrystallisation

Purpose

Solid products obtained after filtration typically contain impurities that need to be removed. Recrystallisation is a purification technique that exploits differences in solubility between the desired product and its impurities. The principle is simple: dissolve the impure solid in a minimum volume of hot solvent, then allow the solution to cool slowly so that pure crystals form, leaving impurities in solution.

Choosing a suitable solvent

Procedure

Dissolving the sample: Pour a quantity of chosen solvent into a conical flask. If the solvent is flammable, warm it gently over a water bath for safety. If using water as the solvent, you can place the conical flask on a tripod and heat with a Bunsen burner.

Transfer your impure solid sample into a second conical flask or beaker. Gradually add the warm solvent to the impure sample, using the minimum volume necessary to completely dissolve the solid. This is crucial - using excess solvent will reduce your final yield of pure product.

Cooling and crystallisation: Once the solid has completely dissolved, remove the heat source and allow the solution to cool slowly to room temperature. As the solution cools, the solubility of your product decreases, causing crystals of the pure product to form in the flask. The impurities, being in lower concentration or having different solubility properties, remain dissolved in the solvent.

Isolating pure crystals: When no more crystals appear to be forming, filter the mixture using filtration under reduced pressure (as described above) to collect the dry crystalline solid. The purified product is now ready for further analysis or use.

Melting point determination

Purpose and significance of melting points

Chemists use melting point determination to assess the purity of solid compounds. A pure organic substance typically has a very sharp melting point, occurring over a narrow range of just one or two degrees Celsius. This narrow melting range indicates high purity.

The melting range is defined as the difference between the temperature at which the sample begins to melt and the temperature at which melting is complete. When a compound contains impurities, two characteristic changes occur:

Sample preparation

Before measuring the melting point, ensure that:

• The sample is completely dry and free-flowing
• You have prepared a sealed capillary tube containing the sample

Preparing the capillary tube: Take a glass capillary tube (or melting point tube) and hold one end in the hot flame of a Bunsen burner. Rotate the tube continuously in the flame until the end seals closed. Allow the tube to cool completely.

Fill the sealed capillary tube with crystals to a depth of approximately 3 mm. This is typically done by pushing the open end of the tube into your solid sample, forcing some crystals into the tube.

Method 1: Using an electrically heated melting point apparatus

This is the most common method used in schools and colleges due to its convenience and accuracy.

Setting up: Place the capillary tube containing your sample into the sample holder of the melting point apparatus. Insert a thermometer ($0-300\,°C$ range) into the thermometer hole of the apparatus.

Recording the melting point: When you observe the solid beginning to melt, record this temperature. Continue heating slowly and record the temperature when melting is complete. The melting range is the difference between these two temperatures.

Ensuring accuracy: To obtain a more accurate melting point, prepare a second sample in a fresh capillary tube. Heat this second sample using the slow heating setting from the start, particularly as you approach the melting point. This slower, more controlled heating provides a more precise determination.

Method 2: Using an oil bath or Thiele tube

This traditional method is still used when electrically heated apparatus is unavailable.

Setting up the apparatus: Set up a Thiele tube or oil bath as shown in your practical manual. Attach the capillary tube containing your sample to a thermometer using a small rubber band, ensuring the sample is level with the thermometer bulb. Insert the thermometer through a hole in the cork (if using a Thiele tube) or clamp it in position (if using an oil bath). The thermometer and capillary tube should dip into the oil.

Heating and observation: Using a micro-burner, gently heat the side arm of the Thiele tube (or the oil bath directly) while continuously observing the solid in the capillary tube. Heat the oil slowly when approaching the melting point - this is essential for accuracy. It's advisable to repeat the determination a second time using even slower heating to ensure you obtain an accurate value.

When the solid begins to melt: Remove the heat source immediately and record the temperature shown on the thermometer. Continue observing until all of the solid has melted completely, then record this final temperature.

Practical considerations and exam tips

Interpreting melting point data:

• A sharp melting point (range of $1-2\,°C$) indicates high purity
• A wide melting range (greater than $5\,°C$) suggests significant impurities present
• A melting point lower than the literature value also indicates impurities

Remember!